In machining processes such as polishing or planarization of thin workpieces, such as silicon substrates or wafers used in integrated circuits, a wafer is disposed between a carrier or pressure plate and a rotatable polishing table carrying on its surface a polishing pad. The pressure plate applies pressure so as to effect removal of a determined amount of oxide coating and to produce a surface of substantially uniform thickness on the wafer.
Generally, the polishing apparatus includes a rigid pressure plate or carrier to which unpolished wafers are adhered, with the wafer surfaces to be polished exposed to a polishing pad which engages the same with polishing pressure. The polishing pad and carrier are then typically both rotated at differential velocities to cause relative lateral motion between the polishing pad and the wafer front side surfaces. An abrasive slurry, such as a colloidal silica slurry, is generally provided at the polishing pad-wafer surface interface during the polishing operation to aid in the polishing.
The preferred type of machine with which the present invention is used includes a rotating polishing wheel which is rotatably driven about a vertical axis. Typically, the polishing wheel comprises a horizontal ceramic or metallic platen covered with a polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives. The polishing pads can be formed of various materials, as is known in the art, and which are available commercially. Typically, the polishing pad is a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation of Scottsdale, Ariz. The hardness and density of the polishing pad is routinely selected based on the type of material that is to be polished. The polishing pad is rotated about a vertical axis and has an annular polishing surface on which the work pieces are placed in confined positions so that movement of the polishing wheel and the superimposed attached polishing pad relative to the work pieces brings about abrasive wear of the latter at their surfaces in engagement with said polishing surface. Of importance in all such machines is the maintenance of the polishing pad surface in planar condition and substantially free of surface irregularities. The polishing pads tend to wear unevenly in the polishing operation and surface irregularities develop therein, and these problems must be corrected.
In wafer planarization processes for oxide layer polishing, the polishing pad may too rapidly become "out of flat" by virtue of a groove called a "track" being formed in the pad. Grooving or tracking of the polishing pad is caused by the leading edge of the wafer dipping and digging into the pad. Abrasive dressing of the pad to remove the track also wears out the polish pad prematurely. In polishing silicon wafers individually secured to power driven flat platens, the wear rate of the polishing pad generally occurs farther out from the center axis.